Negatively charged excitons (X^-) and D^- triplet transitions in GaAs/Al_0.3Ga_0.7As multiple quantum wells

From a combination of low-temperature photoluminescence (PL) and far-infrared magnetospectroscopy on several GaAs/AlGaAs multiple quantum well samples with different donor doping (well only, barrier only, both well and barrier), we have identified a recombination line due to negatively charged excitons (X^-). We have also studied the effects of excess free electrons in the wells on the X^- line. Magneto-PL for low-density barrier-only doped samples shows both free exciton and X^- recombination lines at all values of field studied. However, for more heavily doped samples the behavior is very different. As magnetic field is increased, three distinct features evolve from the broad free carrier recombination lines. At low fields all three features are Landau level recombination lines. At a field corresponding to filling factor ν = 2 the lowest energy line undergoes a discontinuous change in slope, and above this field it evolves into the X^- line. In related far-infrared magnetospectroscopy studies we have made a clear identification of one of the predicted negative donor ion (D^-) triplet transitions.